I too am having a problem with iphone users being able to send me text messages on my Android phone because I made the mistake of trying an iphone for a couple of months.
I started with an Android phone then about a year later, deactivated it and activated my son's old iphone 4s. I didn't have to turn off any messaging or and features on the Android and other then not being able to transfer my contacts, everything else seemed to go seemlessly.
After 2 months I decided I didn't like the iphone so I reversed the process, never in a million years thinking I'd have to unsubscribe to anything or turn anything off. I did both switches with the help of my cell phone carrier.
It wasn't until a few weeks later that I happened to turn on the iphone because I thought I'd still use it to take pictures and noticed an iphone user had sent me a text that day and it never came through on my activated Android phone.
I started searching for help in how to resolve this and saw where it said you should unregister your phone at the apple site but my phone wasn't even listed, however the ipad I no longer had was listed along with the macbook pro I stopped using. I unregistered the ipad and still looked for a solution to the text messaging issue.
The next day I went to my cell phone carrier with both phones and they turned off imessage thinking that would help.
It didn't.
Ironically enough, I have been able to get text messages from 2 different iphone users. I think they both are still using the previous operating system but my friends with the new operating system can't send me a text. Now they're not even coming to the iphone since imessage is turned off.
My next step was to change my password. (one of the other tips) This didn't help but it did now give iphone users a message saying they couldn't deliver the text. If they clicked the exclamation mark, they could resend it as a text and it would go through.
This was not a complete solution so I called the tech department for my cell phone carrier and they said it's an on going problem that they've been working to resolve with Apple.
They said Apple had to take my number out of their portal and they were going to patch me through to an Apple rep to help fix the problem.
According to the Apple rep, my number was taken out of their registry the day I unregistered the ipad so I'm thinking that's how the number got removed.
They also said that eventually this would all straighten itself out but in order to get it fixed immediately, my iphone friends had to delete all text messages from me and delete my contact information then put my contact information back in.
My son did this with his iphone and then tried to send me a text. This time he got an error but was able to resend it as a text so obviously the solution didn't work or didn't immediately work.
I decided to try unregistering my macbook pro and see if that would help but before testing it out, I found this post where it said to hold down the bubble after typing your text and then say send as text. I held down the bubble but I didn't get any options however the text turned to a green bubble and got sent. I tried just sending another text without doing anything special and it too went through. I don't know if it's fixed or what might have fixed it but those are all the steps I took to try and fix this.
Now I'm hoping my iphone friends witll delete me from their contacts and delete my text messages and see if by doing that and reentering my information, they'll be able to send me text messages without any glitch.